The present invention generally relates to an atrial defibrillator and method for applying cardioverting electrical energy to the atria of a human heart in need of cardioversion. The present invention is more particularly directed to a fully automatic implantable atrial defibrillator which exhibits improved safety by reducing the potential risk of induced ventricular fibrillation which may result from the delivery of cardioverting electrical energy to the atria. More specifically, the atrial defibrillator and method of the present invention returns the heart gradually to a normal cardiac rate by pacing the ventricles at gradually decreasing cardiac rates until a preselected normal cardiac rate is obtained after applying cardioverting electrical energy to the atria.
Atrial fibrillation is probably the most common cardiac arrythmia. Although it is not usually a life threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient b way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected ventricular electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistent to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide patients suffering from occurrences of atrial fibrillation with relief. Unfortunately, to the detriment of such patients, none of these atrial defibrillators have become a commercial reality.
Implantable atrial defibrillators proposed in the past have exhibited a number of disadvantages which probably has precluded these defibrillators from becoming a commercial reality. Two such proposed defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these proposed defibrillators require the patient to recognize the symptoms of atrial fibrillation with one defibrillator requiring a visit to a physician to activate the defibrillator and the other defibrillator requiring the patient to activate the defibrillator with an external magnet.
Improved atrial defibrillators and lead systems which exhibit both automatic operation and improved safety are fully described in copending U.S. application Ser. No. 07/685,130, filed Apr. 12, 1991, in the names of John M. Adams and Clifton A. Alferness for IMPROVED ATRIAL DEFIBRILLATOR AND METHOD and U.S. application Ser. No. 07/856,514, filed Mar. 24, 1992, in the names of John M. Adams, Clifton A. Alferness, and Paul E. Kreyenhagen for IMPROVED ATRIAL DEFIBRILLATOR, LEAD SYSTEMS, AND METHOD, which applications are assigned to the assignee of the present invention an incorporated herein by reference. As disclosed in the aforementioned referenced applications, synchronizing the delivery of the defibrillating or cardioverting electrical energy to the atria with a ventricular electrical activation (R wave) of the heart has been considered important to avoid cardioverting the heart during the heart's vulnerable period or T wave to thus prevent induced ventricular fibrillation. Ventricular fibrillation is a fatal arrythmia which can be caused by electrical energy being delivered to the heart at the wrong time in the cardiac cycle, such as during the T wave of the cycle. The atrial defibrillators of the aforementioned referenced applications exhibit improved safety from inducing ventricular fibrillation by sensing ventricular activations of the heart in a manner which avoids detecting noise as ventricular electrical activations for generating reliable synchronization signals. Hence, these implantable atrial defibrillators, by providing such noise immunity in R wave detection assure reliable synchronization.
Another measure for reducing the risk of inducing ventricular fibrillation during the delivery of cardioverting electrical energy to the atria of the heart employed by the defibrillators of the aforementioned referenced applications is the reduction of the amount of the electrical energy which is passed through the ventricles during cardioversion of the atria. This is achieved by locating the cardioverting electrodes in or near the heart to provide a cardioverting energy path which confines most of the cardioverting electrical energy to the atria of the heart.
It has also been observed that during episodes of atrial fibrillation, the cardiac rate often increases to a high rate and/or becomes extremely variable. At high cardiac rates, the R wave of each cardiac cycle becomes closely spaced to the T wave of the previous cardiac cycle. This may create a condition known in the art as an "R on T" condition which is believed to contribute to induced ventricular fibrillation if the atria are cardioverted in synchronism with an R wave close to the preceding T wave. For highly variable cardiac rates, a long cardiac cycle can be followed by a relatively short cardiac cycle. This condition is believed to cause a dispersion of refractoriness and also can result in a vulnerable R on T condition.
To avoid applying the cardioverting electrical energy to the atria during a vulnerable R on T condition, an atrial defibrillator fully described in copending application U.S. Ser. No. 07/965,168, filed concurrently herewith in the names of John M. Adams, Clifton A. Alferness, Kenneth Ross Infinger and Yixuan Jin and entitled ATRIAL DEFIBRILLATOR AND METHOD FOR PROVIDING SYNCHRONIZED DELAYED CARDIOVERSION, which application in incorporated herein by reference, delays the delivery of the cardioverting electrical energy to the atria until after an R wave is detected. This assures that the cardioverting electrical energy is not applied in synchronism with an R wave which is closely adjacent to the T wave of an immediately preceding cardiac cycle.
It has also been observed that after successful cardioversion of the atria, the cardiac rate of the heart will suddenly shift from the aforementioned high rate to a normal sinus rate or even still a lower rate if there is temporary sinoatrial node or atrioventricular node dysfunction resulting from the cardioversion of the atria. Since at high cardiac rates the heart provides low cardiac output due to lowered pumping efficiency, the sudden reduction in cardiac rate could potentially result in the patient experiencing a spell of dizziness. In addition, the sudden reduction in cardiac rate can also lead to dispersion of refractoriness which, if it occurs together with an R on T condition, can render the heart more vulnerable to induced ventricular fibrillation. Hence, such a sudden reduction in cardiac rate following successful cardioversion of the atria may be undesirable.
The atrial defibrillator and method of the present invention overcomes the aforementioned problems connected with the sudden reduction in cardiac rate following successful cardioversion or defibrillation of the atria by gradually returning the cardiac rate to a preselected normal rate after the cardioverting electrical energy is applied to the atria. As will be seen hereinafter, this is accomplished by pacing the ventricles of the heart at gradually decreasing cardiac rate from a base rate determined prior to cardioversion to a final normal rate lower than the base rate after the cardioverting or defibrillating electrical energy is applied to the atria of the heart. Furthermore, pacing of the ventricles is preferably performed in the demand mode, known in the art as the VVI mode, to permit the heart to gradually return to the normal cardiac rate on its own if it is able to do so.